Exposure to electromagnetic fields can cause interference or damage to electrical equipment, causing that equipment to malfunction or rendering it nonoperational. For example, electrical equipment can be damaged or otherwise fail in the event of a strong electromagnetic pulse (EMP) or intentional electromagnetic interference event (IEMI) is experienced.
EMP/IEMI events typically take one of two forms. First, high electric field events correspond to short-duration, high voltage events (e.g., electric fields up to and exceeding 100 kilovolts per meter), and typically are of the form of short pulses of narrow-band or distributed signals (e.g., in the frequency range of 14 kHz to 10 GHz). These types of events typically generate high voltage differences in equipment, leading to high induced currents and burnout of electrical components, for example integrated circuits or solid state detector arrays. Second, low field events (e.g., events in the range of 0.01 to 10 volts per meter) are indications of changing electromagnetic environments below the high field damaging environments, but still of interest in certain applications.
Enclosures designed to protect against EMP/IEMI events are generally required to have substantial shielding properties to prevent electromagnetic signals from reaching an interior of those enclosures. However, it can be difficult to transmit optical (visible or infrared) images or energy from an external source into an interior of a shielded enclosure without causing the enclosure to be susceptible to microwave or RF electromagnetic energy entering the aperture of the enclosure through which the optical energy enters the interior of the enclosure. For these and other reasons, improvements are desirable.